Battery-driven devices such as mobile phones, personal digital assistants (PDAs), note-type personal computers, portable audio players include a rechargeable secondary battery and a charging circuit for charging the battery. There is a known charging circuit that charges a secondary battery based on a DC voltage supplied from a universal serial bus (USB) host (e.g., host adaptor or source device) via a USB cable.
Currently, a charging circuit installed in a mobile device is compliant with the so-called USB battery charging specification (hereinafter referred to as BC specification). There are known several types of hosts. The BC specification, Revision 1.2 (“BC 1.2”) defines a standard downstream port (SDP), a dedicated charging port (DCP), and a charging downstream port (CDP) as types of chargers. BC 1.2 also defines currents that the hosts can supply (current capacities) for each type of the chargers. Specifically, 1,500 mA is defined in the DCP and CDP, while 100 mA, 500 mA, 900 mA, etc. are defined depending on USB versions in the SDP.
As the next generation technique and system for charging a secondary battery using a USB, the so-called USB power delivery specification (hereinafter referred to as PD specification) is under development. According to the PD specification, power that can be supplied is greatly increased up to 100 W from 7.5 W in the BC specification. Specifically, the PD specification allows for the supply of 5 V or greater (more specifically 12 V and 20 V) as a USB bus voltage, and the supply of a larger current (more specifically 2 A, 3 A, and 5 A) than in the BC specification as a charging current.
In relation to overcurrent protection, the USB-PD specification defines a peak current requirement. According to the peak current requirement, even though a supply current IOUT exceeds a threshold value IOC for overcurrent protection, power keeps being supplied on the condition that the percentage of the increased supply current IOUT is less than a predetermined value α and the supply current IOUT exceeds the threshold value IOC for less than a predetermined time period TIMAX.
FIG. 1 describes a circuit diagram of a conventional overcurrent detection circuit 100r. The overcurrent detection circuit 100r includes a detection resistor R1, a switch SW1, a current monitoring unit 20r, and a mask circuit 26. The current monitoring unit 20r may monitor a voltage drop (detected voltage) Vs across the detection resistor R1 and asserts an overcurrent detection signal S1 if the detected voltage Vs exceeds a predetermined threshold value. For example, the current monitoring unit 20r includes a sense amplifier 22 for amplifying the voltage drop across the detection resistor R1 and a comparator for comparing an output voltage from the sense amplifier 22 with a threshold value VTH. The mask circuit 26 may mask an overcurrent detection (assertion of the overcurrent detection signal S1) for an extremely short period of time in order to avoid that an overcurrent state is erroneously detected due to a noise. The mask circuit 26 is configured with an analog filter, a digital timer, etc.
In the overcurrent detection circuit 110r of FIG. 1, the masking time of the mask circuit 26 was set to a predetermined time TIMAX, to see if it meets the USB-PD specification. It was seen that during the masking time, the overcurrent detection circuit 100r could not detect an overcurrent state. Accordingly, in case that an output current IOUT exceeds a threshold value IOC for overcurrent protection during the masking time and a percentage of the increased output current IOUT exceeds a predetermined value α, the overcurrent detection circuit 100r could not determine that it is in an overcurrent state.